1. Field of the Invention
This invention relates to control circuits which provide voltage regulation for switching power supplies, and in particular to an improved, less costly control circuit.
2. Description of Prior Art
My invention may be incorporated in a variety of types of non-symmetrical power supplies where current is loaded into a magnetic device, such as a transformer or an inductor, in one direction only. A representative prior art non-symmetrical switching power supply is shown in FIG. 1a. FIG. 1a is a flyback switching power supply by Motorola, which can be found in the book Motorola Power MOSFET Transistor Data, p. A-72. The operation of a flyback switching power supply similar to that shown in FIG. 1a is described in detail in a copending application entitled, "An Improved Voltage Regulator Circuit", by David T. Carroll, Ser. No. 07/137,787, filed Dec. 23, 1987, submitted herewith and herein incorporated by reference. The operation of the flyback switching power supply of FIG. 1a is as follows. DC voltage V.sub.in is applied to the non-dot-end (dots indicate the relative polarity of a voltage across a winding) of primary winding W1 of transformer T1. Switching transistor Q1, coupled between the dot-end of winding W1 and ground GND1, is controlled by pulsewidth modulator 10 so that when transistor Q1 is on, current flows through winding W1, and, when transistor Q1 is off, current ceases to flow through winding W1. As seen by the locations of the dot-ends of windings W2, W3, and W4 of transformer T1, the polarities of the voltages across windings W2, W3 and W4 are opposite that of the polarity of voltage V.sub.in across winding W1. Thus, when transistor Q1 is on and V.sub.in is applied across winding W1, the voltage at the dot-end of windings W2, W3, and W4 will be negative with respect to their respective grounds. Thus, when V.sub.in is applied across winding W1, diode D1, whose anode is connected to the dot-end of secondary winding W2 of transformer T1, is reverse biased along with diodes D2 and D3 whose anodes are similarly connected to the dot-ends of secondary winding W3 and auxiliary winding W4. Transformer T1 now linearly charges up due to the current through winding W1 and stores energy in the form of a magnetic field in the core of transformer T1. During this time, the proper output voltage V.sub.out and necessary current are being supplied to the output load by output filtering capacitor C1. Voltage V.sub.out is applied to error amplifier 12, which compares a voltage corresponding to V.sub.out to a reference voltage and supplies an error signal to coupler 13, which may be an opto-coupler for providing isolation between the input and output sections of the power supply. Coupler 13 then supplies a corresponding feedback signal V.sub.fs to pulsewidth modulator 10, which, if necessary, adjusts the duty cycle (i.e., time on/time off) of a fixed frequency quasi-squarewave control signal applied to the control gate of switching transistor Q1 to adjust the amplitude of V.sub.out.
When transistor Q1 is switched off by control of pulsewidth modulator 10, the magnetic field in the core of transformer T1 tries to collapse, and in doing so reverses the voltage polarities on all windings. Thus, the voltage at the dot-end of secondary winding W2 is now positive with respect to groun GND2 and rises until it forward biases diode D1, where it becomes clamped at V.sub.out +0.7 volts. The energy stored in transformer T1 is now released into the output load at a current required by the load and recharges capacitor C1 to the desired voltage.
A block diagram of prior art pulsewidth modulator 10, shown in the above mentioned Motorola Data Book on p. A-74, is shown in FIG. 1b. This pulsewidth modulator requires oscillator 16 to generate a fixed frequency sawtooth waveform, shown in FIG. 1b, graph A. This sawtooth waveform is applied to the plus (+) terminals of comparators 18 and 20. Feedback signal V.sub.fs from coupler 13 is applied to the minus (-) terminal of comparator 20 so that, as shown in FIG. 1b, graph B, when the amplitude of the sawtooth waveform exceeds V.sub.fs, the output of comparator 20 goes high. This comparator 20 output signal is then OR'ed by OR gate 22 with the output of comparator 18, whose only function is to provide an initial signal for startup of the power supply. The resulting quasi-squarewave output of OR gate 22 is then amplified by transistor Q2 and applied to the gate of switching transistor Q1, which switches on and off in response to the output of OR gate 22 with the proper duty cycle to achieve the desired V.sub.out. Thus, when V.sub.out is lower than a predetermined level, the on-time of switching transistor Q1 increases, raising V.sub.out to the desired level. More detail on these oscillators may be found in the book Switching and Linear Power Supply, Power Converter Design, by Abraham Pressman, Hayden Book Company, N.J., 1977, pp. 316-319.